1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
Electrophotography is generally inferior to offset printing in image stability in image formation. Various techniques, thus, have been employed so as to bring the image stability in the electrophotography close to that in the offset printing.
One of the different points between the electrophotography and the offset printing is gradation expression. The electrophotography expresses a half tone gradation in a pseudo manner by performing image processing such as dither processing using dither patterns such as a line screen and a dot screen on image data.
In the electrophotography, the dot screen achieves high image stability in low gradation values and high gradation values, but achieves low image stability in intermediate gradation values. In contrast, in the electrophotography, the line screen achieves high image stability in the intermediate gradation values, but achieves low image stability in the high gradation values. As a result, high image stability is not achieved in a certain gradation even if both of the dot screen and the line screen are used. For solving such a problem, a hybrid screen technique is known that uses the dot screen and the line screen by switching them in accordance with a range of gradation values in gradation processing, for example.
For example, Japanese Patent Application Laid-open No. 2012-3180 discloses a technique in which the line screen is used in a range of intermediate gradation values while a void dot screen is used in a range of high gradation values in order to use advantages of the dot screen and the line screen. The line screen has a disadvantage in that the width of a toner non-adhesion region (i.e., a region between lines) is reduced with an increase in gradation value, thereby causing a toner adhesion state in the non-adhesion region to be unstable. The technique disclosed in Japanese Patent Application Laid-open No. 2012-3180, thus, improves gradation performance in a range from an intermediate gradation to a high gradation by switching the dither pattern from the line screen to the void dot screen.
In such a conventional technique, however, image quality such as color stability deteriorates in a range of the gradation values in the switching from the line screen to the void dot screen in some cases. In addition, the conventional technique needs to store two different dither patterns, thereby requiring for a memory storing the dither patterns to have a large storage capacity. Furthermore, the conventional technique needs to perform dot re-arrangement processing for each dot array at the adjacent regions of the two different types of dither patterns, thereby requiring a time for the image processing.
The conventional technique does not achieve high image stability in a range of the gradation values in the switching from the line screen to the void dot screen in some cases. When an image defect such as banding unique to an apparatus occurs near the range of the gradation values, the low image stability causes the image defect to be more conspicuous in some cases.
In view of the above, there is a need to improve the image stability in a range from an intermediate gradation to a high gradation, and prevent an increase in memory capacity, an increase in image processing time, and an image defect from being more conspicuous even when both of the line screen and the void dot screen are used.